This invention relates to an improved process of preparing a fluorinated carbonyl compound, which is expressed by the general formula (II), by dehydrating decomposition of a gem-diol, hemi-acetal or hemi-ketal, which is expressed by the general formula (I), using sulfuric acid as a decomposing and dehydrating agent. ##STR2## where A represents H or CF.sub.3, and R represents H, CH.sub.3 or C.sub.2 H.sub.5. ##STR3## where A represents H or CF.sub.3.
For the purpose of storing a fluorinated carbonyl compound which is expressed by the general formula (II) and has a boiling point below room temperature in a stable state, it has been put into practice to convert the carbonyl compound into a gem-diol, hemi-acetal or hemi-ketal of the general formula (I). The alcoholizing or acetalizing reaction is represented by the following equation (1). ##STR4##
Since the reaction of Equation (1) is reversible, the carbonyl compound (II) can be isolated in the initial form by the dehydrating decomposition of the gem-diol, hemi-acetal or hemi-ketal (I) preparatory to the use of the carbonyl compound as a starting material in various synthesis reactions. The decomposition of the compound (I) as the reverse reaction of Equation (1) is customarily accomplished by making the compound (I) contact with concentrated sulfuric acid at a relatively high temperature, i.e. at a temperature above 90.degree. C. However, it is a disadvantage of this method from an economical point of view that a very large amount of concentrated sulfuric acid needs to be used.
Hexafluoroacetone (CF.sub.3).sub.2 C.dbd.O (abbreviated to HFA) can be named as a typical example of fluorinated carbonyl compounds of the formula (II). HFA is very high in reactivities and has wide uses as intermediates of medicines and agricultural chemicals. Under the atmospheric pressure, HFA is a gaseous compound having a boiling point of -28.degree. C. and reacts rapidly with water to form monohydrate (will be abbreviated to HFA-1W), which is regarded as a gem-diol (CF.sub.3).sub.2 C(OH).sub.2, as represented by the following equation. ##STR5## HFA-1W is a solid in the form of white needle-shaped crystals having a melting point of 47.degree. C. and readily dissolves in water. When HFA-1W is dissolved in such an amount of water that the mole ratio of water to HFA is about 3, the solution becomes an azeotropic composition (will be represented by HFA-3W) of which the boiling point is 106.degree. C. under the atmospheric pressure.
Since HFA is a noxious gas having a characteristically irritating smell and it is difficult to handle gaseous HFA as an industrial material, it is usual to transport and store HFA in the state of HFA-3W which is a liquid convenient for handling.
Preparatory to a reaction process using HFA, the stored HAF-3W can be decomposed and dehydrated with concentrated sulfuric acid usually at temperatures in the range from 90.degree. to 120.degree. C. It is ensured that the reaction proceeds in the presence of concentrated sulfuric acid, firstly because the solubility of anhydrous HFA in sulfuric acid increases as the concentration of sulfuric acid lowers and secondly because lowering in the concentration of sulfuric acid causes a rise in the vapor pressure of water on sulfuric acid so that the intended dehydration becomes incomplete with the result of precipitation of the gem-diol or HAF-1W on the wall of the reactor. It is conceivable to lower the reaction temperature to thereby lower the vapor pressure of water, but it is impracticable to lower the reaction temperature below 90.degree. C. from an industrial point of view because of extreme lowering of the rate of the decomposition reaction. For these reasons the reaction is carried out with best care not to lower the concentration of sulfuric acid, and therefore a very large amount of sulfuric acid must be used.